1. Field of the Invention
The present invention relates to a real time contamination monitor, more specifically, a contamination monitor which is capable of measuring contamination at the molecular level.
2. Description of the Related Art
The cleanliness requirements for the manufacture and operation of sophisticated technical systems are becoming ever more stringent. This is especially true in the manufacturing processes involved in microelectronics, high precision optics, as well as in the preparation of these types of systems for flight of spacecraft. It is no longer sufficient just to maintain a certain level of particle matter in a work environment, as has been the practice for several decades; it is becoming clear that contamination on a molecular level can create serious manufacturing and operational problems.
It is well known that all materials and most activities emanate gases or small aerosols by diffusion and desorption. The term "contamination" is applied when the emitted gas or aerosol impinges and condenses on a subject surface. Contamination of a "clean" surface orginates from two main sources: activities or processes in the clean work area and from the materials used in the construction of the article itself (self-induced contamination).
Non-volatile residue, (NVR), sometimes referred to as molecular contamination, on critical surfaces surrounding space structures have been shown to have a dramatic impact on the ability to perform optical measurements from platforms based in space with the particulate and NVR contamination originating primarily from pre-launch operations. Molecular deposition on surfaces affects the thermal balance of a spacecraft scheduled for a long duration mission since the absorptance and emittance of the thermal control panels are adversely affected. Any optical surface (such as windows or mirrors) is degraded by molecular depositions and particulates. Condensed fills of contaminants on the order of 10 angstroms thick degrade the efficiency and operation of the optical components. Therefore, a real-time measurement of NVR is required to assure that critical components are fully operational and not subjected to high levels of contaminants during payload processing and storage.
The pre-launch NVR contamination problem is even greater with the proposed Space Station since the large surface area of this structure would contribute significantly to the molecular contamination and could jeopardize the operation of numerous scientific instruments planned for this mission.
The currently accepted method of measuring NVR is the use of witness plates to collect the NVR over a time period up to several weeks. The major drawback to this technique is that the NVR contamination is integrated which precludes the real-time identification of a contamination event and the ability to curtail activities in the area which causes contamination. The method is also time consuming and tedious since a technician must wash the plates with a suitable solvent to remove the residue, filter the extract and place it in a pre-weighed dish which is then brought to dryness. The weight of the remaining residue is considered NVR and is expressed as mg/0.1 m.sup.2 /month. A sensitive real-time NVR monitor would be a valuable instrument to reduce the overall level of contamination since activities which generate high levels of NVR would be detected in real-time and corrective measures can be taken in a timely manner.
More recently, the piezoelectric crystal microbalance has been used for the measurement of surface deposition on the particle level. Piezoelectric crystals in this category have operated in the bulk-vibration mode wherein the entire body of the crystal is driven electrically into resonance. The piezoelectric crystal operates as a microbalance by the de-tuning of the crystal's resonant frequency when mass is added to its surface.
U.S. Pat. No. 4,561,286 issued to Selker, et at. discloses such a bulk piezoelectric crystal microbalance. The bulk-vibration method requires the placement of the resonating electrodes on the opposite side of the bulk crystal, wherein the distance between the electrodes, i.e., the thickness of the crystal, defines the resonating frequency of the crystal. Therefore, the resonant frequency of a bulk vibration crystal is inversely proportional to the crystal thickness. The limit of the resonant frequency that can be obtained with a bulk mode crystal is approximately 15 MHz, because a thinner crystal would be manageable and therefore is not produced. Since the change in mass detectable by the crystal is proportional to the square of its frequency, the limit of mass resolution in the bulk vibration mode is typically on the order of 10.sup.-9 to 10.sup.-8 g-cm.sup.-2. This level of mass resolution is sufficient to detect contamination at a particle level but is not fine enough to detect contamination at a molecular level.
Therefore, there exists a need for a real time piezoelectric monitor to measure contamination at a molecular level which can detect changes in mass due to molecular contamination on the order of 10.sup.-11 to 10.sup.-13 g-cm.sup.-2 levels.